Anaerobic digestion has been known to stabilize sludge and other predominantly organic materials, and usable product gas, of varying composition, has been obtained from such anaerobic digestion processes. The organic feed mixture which provides the substrate for anaerobic biodegradation can comprise a wide variety of organic carbon sources, ranging from raw sewage sludge to municipal refuse, or biomass material such as plants and crop wastes. The process of anaerobic digestion degrades any of these organic carbonaceous materials, under appropriate operating conditions, to product gas which contains hydrogen and methane gases.
Anaerobic digestion uses a consortium of natural bacteria to degrade and then convert an organic substrate into a mixture of methane and carbon dioxide. The existing anaerobic digestion systems for organic substrate digestion can be separated into two major types, one phase systems and two phase systems. Existing one phase systems include the batch digester, completely mixed digester and the plug flow digester. These one phase systems, in which the organic substrate and the microorganisms are housed together are easy to operate and of low cost. Completely mixed digesters and plug flow digesters require continuous handling of feedstock and do not operate in batch mode. Further, the biogas produced in one phase systems consists primarily of carbon dioxide in the early stages of digestion. The high carbon dioxide content of the biogas is attributable to the slow growth of the methanogenic microorganisms and their inhibition by high concentrations of volatile fatty acids (VFAs). In order to reduce the inhibition of the microorganisms by the VFAs, the two phase digester has been introduced.
Separated two phase anaerobic digestion systems have been found to enhance the conversion efficiency, such as described in Pohland and Ghosh, Biotechnol. and Bio-eng. Symp. No. 2, 85-106 (1971), John Wiley and Sons, Inc. and by the same authors in Environmental Letters, 1: 255-266 (1971). A typical two phase anaerobic digester system comprises a hydrolytic and a biogasification reactor. The acid phase digester is usually designed as a solid-bed batch reactor where solid waste is housed and leached soluble compounds are collected. In the acid first phase, the microbial population and operating conditions are selected to promote the conversion of organic matter to soluble compounds of lower molecular weight, primarily VFAs. The liquid and solid effluent from the acid phase is conveyed to a biogasification second phase, where methanogenic organisms convert the VFAs to product gas that is composed primarily of methane and carbon dioxide. Product gas is removed from the biogasification reactor and processed, or scrubbed, to separate the methane component that is drawn off as pipeline gas.
Anaerobic digestion of solid waste, particularly agricultural residues and municipal organic solid wastes, is a promising technique for both generating energy and reducing the volume of waste which must be disposed of. The energy generated can be significant. For example, the energy content of a pound of rice straw is about 6,500 Btu (British Thermal Units), and the energy stored in the straw by growing crop each year in the Sacramento Valley is 1.95×1012 Btu. One ton of food leftovers collected from restaurants could be used to produce 2.2-2.7×106 Btu biogas energy (Zhang et al., 2007), thus, it is realistic to consider agricultural residues and municipal organic wastes as a renewable resource for energy generation.
Anaerobic digestion is an enhanced biodegradation process that offers a promising alternative approach for helping solve problems caused by agricultural waste such as the imminent rice straw disposal problems in concentrated rice production regions such as California. It also offers a solution in reducing the greenhouse gas emissions from landfills where most of municipal organic wastes are disposed of. Anaerobic digestion uses a consortium of microorganisms to degrade and then convert a large portion of organic waste into biogas, which is a mixture of hydrogen, methane and carbon dioxide. If captured, biogas can be utilized as a clean fuel for heat and power generation or transportation.
However, the previously developed two phase anaerobic systems are not efficient systems. First, hydrogen produced by certain microorganism during the breakdown of organic matter is consumed by other microorganisms in the system, with the result that only methane is produced. It would more efficient if the microorganisms in the hydrolysis reactors are selected and environmental conditions are controlled to allow production and release of hydrogen in the first phase prior to methane production in the second phase. Second, the VFAs in the various hydrolysis reactors are not equilibrated when they enter in the biogasification reactor. Therefore, the methane-producing bacteria in the biogasification reactor do not react efficiently with the VFAs, resulting in inefficient gas production. Third, in the previously developed systems, a perforated plate is installed inside the hydrolysis reactor to separate liquid from solids and the outlet of the hydrolysis reactor is located at the bottom of the reactor to allow the decanting of liquid from the reactor. Such a design allows only a portion of the hydrolysis reactor to be utilized for the reaction, resulting in lower efficiency. Quite surprisingly, the present invention provides methods and devices which solve these problems.